Turbine engine shroud segment

ABSTRACT

A turbine engine shroud segment comprises a body including an outer surface away from which an integral attachment system projects. The attachment system comprises a plurality of at least two axially spaced apart circumferentially extending rows of discrete projection hook segments to interrupt a potential hoop stress path through the attachment system. Each projection hook segment includes a segment support surface aligned circumferentially and radially with other segment support surfaces in a row.

The Government has rights in this invention pursuant to Contract No.F33615-97-C-2778-7UW awarded by the Department of Air Force.

BACKGROUND OF THE INVENTION

This invention relates generally to turbine engine shroud segmentsincluding a surface exposed to elevated temperature engine gas flow.More particularly, it relates to air cooled gas turbine engine shroudsegments, for example used in the turbine section of a gas turbineengine, and made of a low ductility material.

A turbine engine shroud segment is a stationary engine componentseparate and distinct from a component that includes an airfoil, forexample a nozzle segment or a stationary blading member. In a gasturbine engine, a plurality of stationary shroud segments is assembledcircumferentially about an axial flow engine axis and radially about andspaced apart from rotating blading members, for example about rotatingturbine blades. Together, such shroud segments define a part of theradial flowpath boundary over the blades. As has been described invarious forms in the gas turbine engine art, it is desirable to maintainthe operating clearance between the tips of the rotating blades and thecooperating, juxtaposed spaced-apart surface of the stationary shroudsegments as close as possible to enhance engine operating efficiency.Typical examples of U.S. patents relating to turbine engine shrouds,shroud segments and such shroud clearance include U.S. Pat. No.5,071,313—Nichols; U.S. Pat. No. 5,074,748—Hagle; U.S. Pat. No.5,127,793—Walker et al.; U.S. Pat. No. 5,562,408—Proctor et al.; andU.S. Pat. No. 6,702,550 B2—Darkins, Jr. et al.

In its function as a flowpath component, the shroud segment must becapable of meeting the design life requirements selected for use in adesigned engine operating temperature and pressure environment. Toenable current materials to operate effectively as a shroud segment inthe strenuous temperature and pressure conditions as exist in theturbine section flowpath of modern gas turbine engines, it has been apractice to provide cooling air to an outer portion of the shroudsegment. However as is well known in the art, for example as describedin some of the above identified patents, provision of such cooling airis at the expense of engine efficiency. Therefore, it is desired toconserve use of cooling air by minimizing leakage into the flowpath ofthe engine of cooling air not intended to be introduced into theflowpath. For example, some forms of shroud segments include coolingpassages intentionally to pass cooling air into the engine flow stream.However, cooling air leakage about edges of a shroud segment can reducedesigned efficiency by wasting cooling airflow.

It has been observed that one source of such segment edge leakage canresult from shroud segment deformation such as deflection or distortion,generally referred to as “chording”. Chording results from a thermaldifferential or gradient between a higher temperature radially innershroud surface and a lower temperature, air cooled shroud outer shroudsurface. At least the radially inner or flowpath surface of a shroud andits segments are arced circumferentially to define a flowpath annularsurface about the rotating tips of the blades. The thermal gradientbetween the inner and outer faces of the shroud, resulting from coolingair impingement on the outer surface, causes the arc of the shroudsegments to chord or tend to straighten out circumferentially. As aresult of chording, the circumferential end portions of the innersurface of the shroud segment tend to move radially outwardly in respectto the middle portion of the segment. If allowed to occur, this type ofaction can increase the tip clearance required to prevent a rub betweenthe blade and the shroud. As a shroud straightens from its originalcurvature, the shroud ends pull away from the intended flowpath andeffectively increase the clearance of the blade. Therefore, for moreefficient engine operation, it is desirable to restrain chording or sealthe gap resulting from chording.

As is well known in the gas turbine engine art, other segment distortionor distortion forces can occur, for example in a high pressure turbine.Such forces are generated by pressure differences acting on a shroudsegment as a result of a relatively high cooling air pressure on aradially outer portion of a shroud segment, opposite a lower flow streampressure which reduces further passing downstream through a turbine. Incertain shroud segment attachment systems that include shroud segmentsupports integral with the shroud segment and generally in the shape ofcircumferential rings or hoops, tensile stresses resulting from suchthermal gradients can develop in a circumferential path through asupport. Such tensile stresses in ring-like structures frequently arereferred to as hoop stresses.

Metallic type materials currently and typically used as shroud segmentsand their supports have mechanical properties including strength andductility sufficiently high to enable the shroud segments to berestrained against such deflection, distortion or excessive hoopstresses resulting from thermal gradients and other forces. Examples ofsuch restraint include the well known side rail type of structure, orthe C-clip type of sealing structure, for example described in the aboveidentified Walker et al patent. That kind of restraint and sealingresults in application of a compressive force at least to one end of theshroud to inhibit chording or other distortion. Other known shroudsegment restraints or attachment systems can result in significant hoopstress development in an integral attachment system.

Current gas turbine engine development has suggested, for use in highertemperature applications such as shroud segments and other components,certain materials having a higher temperature capability than themetallic type materials currently in use. However such materials, formsof which are referred to commercially as a ceramic matrix composite(CMC), have mechanical properties that must be considered during designand application of an article such as a shroud segment. For example, asdiscussed below, CMC type materials have relatively low tensileductility or low strain to failure when compared with metallicmaterials. Also, CMC type materials have a coefficient of thermalexpansion (CTE) in the range of about 1.5-5 microinch/inch/° F.,significantly different from commercial metal alloys used as restrainingsupports or hangers for shrouds of CMC type materials. Such metal alloystypically have a CTE in the range of about 7-10 microinch/inch/° F.Therefore, if a CMC type of shroud segment is restrained and cooled onone surface during operation, forces or stresses can be developed in CMCtype segment sufficient to cause failure of the segment or its integralattachment system.

Generally, commercially available CMC materials include a ceramic typefiber for example SiC, forms of which are coated with a compliantmaterial such as BN. The fibers are carried in a ceramic type matrix,one form of which is SiC. Typically, CMC type materials have a roomtemperature tensile ductility of no greater than about 1%, herein usedto define and mean a low tensile ductility material. Generally CMC typematerials have a room temperature tensile ductility in the range ofabout 0.4-0.7%. This is compared with metallic shroud and/or supportingstructure or hanger materials having a room temperature tensileductility of at least about 5%, for example in the range of about 5-15%.

Shroud segments made from CMC type materials, although having certainhigher temperature capabilities than those of a metallic type material,cannot tolerate the above described and currently used type ofcompressive force or similar restraint force against chording. Neithercan they withstand a stress rising type of feature, for example oneprovided at a relatively small bent or filleted surface area, withoutsustaining damage or fracture typically experienced by ceramic typematerials. Furthermore, manufacture of articles from CMC materialslimits the bending of the SiC fibers about such a relatively tightfillet to avoid fracture of the relatively brittle ceramic type fibersin the ceramic matrix.

In some applications and embodiments using CMC materials as a shroudsegment, high pressure loading on a shroud segment integral attachmentsystem, coupled with radial temperature gradients, can develop hoopstress amounts in a substantially continuous path through suchattachment system that can result in failure of the attachment system.Provision of a shroud segment of such a low ductility material, thatincludes a shroud segment attachment system that interrupts such a hoopstress path through the system, would enable advantageous use of thehigher temperature capability of CMC material for that purpose.

BRIEF SUMMARY OF THE INVENTION

The present invention provides a shroud segment for use in a turbineengine, for example in a gas turbine engine turbine shroud assembly,comprising a body including a body inner surface and a body outersurface spaced apart from the body inner surface. The body extendsbetween spaced-apart body first and second axial edge portions andspaced-apart body first and second circumferential edge portions. Theshroud segment includes an attachment system comprising projectionhooks, for carrying the shroud segment, integral with and projectingaway from the body outer surface. Each projection hook comprises a hookarm extending away from the body outer surface and a hook end portionextending axially and including a segment support surface of selectedsupport surface shape facing toward the body outer surface.

According to embodiments of the present invention, to interrupt or cut apotential hoop stress path through the attachment system of integralprojection hooks and to reduce hoop stress induced by a radialtemperature gradient in the projection hooks of the attachment system,there is provided a plurality of individual, discrete projection hooksegments. Such projection hook segments are in at least two axiallyspaced-apart circumferentially extending rows defining the attachmentsystem. Each row comprises a plurality of the discrete projection hooksegments spaced-apart circumferentially along the body outer surface atleast partially between the first and second circumferential edgeportions. Each hook end portion of each projection hook segment in a rowfaces an axial edge portion, and each segment support surface of eachprojection hook is aligned circumferentially and radially with othersegment support surfaces in a row.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic, perspective view of an embodiment of a shroudsegment according to the present invention, including two rows ofprojection hook segments.

FIG. 2 is a diagrammatic view from a circumferential end of the shroudsegment of FIG. 1, viewed along lines 2-2.

FIG. 3 is a diagrammatic view, similar to FIG. 2, from a circumferentialend of another form of shroud segment including three rows of projectionhook segments.

FIG. 4 is a diagrammatic, perspective, partially fragmentary view ofstill another embodiment of a shroud segment according to the presentinvention, including two rows of projection hook segments.

FIG. 5 is a diagrammatic, fragmentary view of a shroud segment similarto that in FIG. 1 viewed from axially aft of the shroud segmentassembled in a turbine engine circumferentially about the axis of theengine.

DETAILED DESCRIPTION OF THE INVENTION

The present invention will be described in connection with an axial flowgas turbine engine for example of the general type shown and describedin the above identified Proctor et al patent. Such an engine comprises,in serial flow communication generally from forward to aft, one or morecompressors, a combustion section, and one or more turbine sectionsdisposed axisymmetrically about a longitudinal engine axis. Accordingly,as used herein, phrases using the term “axially”, for example “axiallyforward” and “axially aft”, refer to relative positions or generaldirections in respect to the engine axis; phrases using forms of theterm “circumferential” refer to circumferential position or directiongenerally about the engine axis; and phrases using forms of the term“radial”, for example “radially inner” and “radially outer”, refer torelative radial position or direction generally from the engine axis.

FIG. 1 is a diagrammatic perspective view of an embodiment of a turbineengine shroud segment according to the present invention. Suchembodiment includes an attachment system that enables carrying of ashroud segment, made of the above described low ductility materials suchas a CMC, in a turbine engine shroud assembly without application ordevelopment of excessive pressure or force to the shroud segment,including excessive hoop stresses. A shroud segment shown generally at10 includes a shroud segment body, shown generally at 12, having a bodyradially inner surface 14 and a body radially outer surface 16 spacedapart from radially inner surface 14. Body 12 extends between spacedapart first and second axial edge portions, respectively 18 and 20, andfirst and second circumferential edge portions, respectively 22 and 24.

Shroud segment 10 includes a plurality of individual, discreteprojection hook segments, shown generally at 26, integral with andextending generally away from body radially outer surface 16. Forcarrying shroud segment 10, projection hook segments 26 comprise ashroud segment attachment system that severs a potential hoop stresspath through such system. In such attachment system, projection hooksegments 26 are spaced-apart and aligned circumferentially along bodyouter surface 16 in a plurality of axially spaced-apart,circumferentially extending rows of projection hook segments. In FIG. 1,such rows comprising the integral attachment system are shown generallyat 28 and 30.

In the drawings, each projection hook segment 26 comprises a hooksegment arm 32 extending away from body outer surface 16 and a hooksegment end portion 34 extending generally axially toward an axial edgeportion 18 or 20. Hook segment end portion 34 includes a segment supportsurface 36 facing toward body outer surface 16 and of a selected supportsurface shape, for example planar or arcuate. In FIG. 1, segment supportsurfaces 36 of projection hook segments 26 in a circumferential row, forexample in row 28 and in row 30, are aligned circumferentially andradially one with another in the row. Segment support surface 36 isshown more clearly in FIGS. 2, 3 and 5. In FIG. 1, all hook segment endportions 34 face toward segment body second axial edge portion 20.

As used in connection with the present invention, the terms “toward” or“away from” in respect to a surface direction means generally andpredominantly in the direction with respect to such surface or member.In the drawings, orientation of shroud segment 10 and its attachmentsystem comprising a plurality of circumferential rows of projection hooksegments are shown in respect to a turbine engine by arrows 38, 40 and42 representing, respectively, the engine circumferential, axial, andradial directions.

FIG. 2 is another diagrammatic view of the embodiment of FIG. 1, alonglines 2-2, toward circumferential edge portion 22 of shroud segment 10of FIG. 1. FIG. 2 shows projection hook segments 26 in circumferentialrow 28 and in circumferential row 30 spaced-apart in axial direction 40with their segment support surfaces 36 aligned respectively incircumferential direction 38 and radial direction 42. In FIG. 2, segmentsupport surfaces are shown to have a planar selected support surfaceshape.

FIG. 3 is a diagrammatic view, similar to FIG. 2 and based on a shroudsegment as described in connection with FIG. 1, toward a circumferentialedge portion 22 of another embodiment of a shroud segment according tothe present invention. In the embodiment of FIG. 3, a plurality ofprojection hook segments 26 is circumferentially spaced-apart in threecircumferentially extending rows represented by 28, 44 and 30,respectively spaced-apart in axial direction 40 aft along body radiallyouter surface 16. As described in connection with FIGS. 1 and 2,projection hook segments 26 in a circumferential row are spaced-apart incircumferential direction 38, with their segment support surfacesaligned in circumferential direction 38 and in radial direction 42. InFIG. 3, all hook end portions 34 face toward second axial edge portion20.

FIG. 4 is a diagrammatic, perspective, partially fragmentary view ofstill another embodiment according to the present invention, includingtwo axially spaced-apart, circumferentially extending rows, first row 28and second row 46, of circumferentially spaced-apart projection hooksegments 26. In this embodiment, hook end portions 34 of projection hooksegments 26 in circumferential first row 28 face toward second axialedge portion 20 while hook end portions 34 of projection hook segments26 in circumferential second row 46 face toward first axial edge portion18. As described above in connection with other embodiments, segmentsupport surfaces 36 in a row are aligned one with another incircumferential direction 38 and in radial direction 42.

The diagrammatic, fragmentary view of FIG. 5 is of a shroud segment,similar to that in FIG. 1 and viewed aft of shroud segment 10 from axialdirection 40, assembled circumferentially adjacent other shroud segments(not shown) as is typical in a turbine engine shroud assembly,circumferentially about engine axis 48. In the embodiment of FIG. 5, theshape of segment support surfaces 36 of each projection hook 26 in acircumferential row is selected to be an arc along a circle, shown inphantom as 50, with radius 52 about engine axis 48.

The present invention provides a shroud segment including an integralattachment system that interrupts a potentially damaging, substantiallycontinuous hoop stress path through the attachment system, enabling usein a turbine engine of a shroud segment made of a low ductilitymaterial. The present invention has been described in connection withspecific examples, materials and structures. However, it should beunderstood that they are intended to be representative of, rather thanin any way limiting on, the scope of the present invention. Thoseskilled in the arts relating to the materials, design, manufacture andassembly of turbine engines will understand that the invention iscapable of variations and modifications without departing from the scopeof the appended claims.

1. A shroud segment, for use in a turbine engine, comprising a bodyincluding a body inner surface and a body outer surface spaced apartfrom the body inner surface, the body extending between spaced apartbody first and second axial edge portions and spaced apart body firstand second circumferential edge portions, the shroud segment including aplurality of spaced apart projection hooks, for carrying the shroudsegment, integral with and projecting away from the body outer surface,each projection hook comprising a hook arm extending away from the bodyouter surface and a hook end portion extending axially and including asegment support surface of selected support surface shape facing towardthe body outer surface, wherein the plurality of projection hookscomprises: at least two axially spaced apart circumferentially extendingrows of discrete projection hook segments, each row comprising aplurality of the discrete hook segments spaced apart circumferentiallyalong the body outer surface at least partially between the first andsecond circumferential edge portions with each hook end portion of eachprojection hook segment in a row facing toward an axial edge portion;and, each segment support surface of each projection hook segment isaligned circumferentially and radially with other segment supportsurfaces in a row.
 2. The shroud segment of claim 1 in which the shroudsegment is made of a low ductility material having a low tensileductility, measured at room temperature to be no greater than about 1%.3. The shroud segment of claim 1 in which the selected support surfaceshape is an arc of a circle about an axis of the turbine engine.
 4. Theshroud segment of claim 1 in which the selected support surface shape isplanar.
 5. The shroud segment of claim 1 in which each hook end portionof each of the discrete projection hook segments faces toward the sameaxial edge portion.
 6. The shroud segment of claim 5 in which theplurality of projection hooks comprises two axially spaced apartcircumferentially extending rows of discrete projection hook segments.7. The shroud segment of claim 5 in which the plurality of projectionhooks comprises three axially spaced apart circumferentially extendingrows of discrete projection hook segments.
 8. The shroud segment ofclaim 1 in which: the plurality of projection hooks comprises at leasttwo axially spaced apart circumferentially extending rows of discreteprojection hook segments; each hook end portion of a first row ofdiscrete projection hook segments facing the second axial edge portion;and, each hook end portion of a second row of discrete projection hooksegments facing the first axial edge portion.